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We extract significant spatially coherent strain variations from horizontal seasonal Global Positioning System (GPS) displacements in the American Southwest. The dilatational strain is largest in northern California with maximum margin-normal contraction and extension in spring and fall, respectively, consistent with the Earth's surface going down and up at those times. The northern California signal has a phase shift with respect to that in southern California and the Great Basin. For northern and southern California the proportion of larger earthquakes are in-phase and the aftershock productivity out of phase with the inferred Coulomb stress on the San Andreas fault system. The intensity of mainshocks is in-phase in the north as well but not in the south. This suggests that a seasonal increase in fault-normal extension may or may not trigger mainshocks, but when an earthquake happens at those times, they grow larger than they otherwise would, which would cause a larger stress reduction and result in fewer aftershocks.

Plain Language Summary The changing amount of water and snow mass that lays on top of the Earth's surface is one possible explanation for observed seasonal variations in seismicity. This hydrological loading would change the state of stress inside the crust minutely with the seasons. We image the seasonal stress variation by using the horizontal seasonal displacements of GPS monuments in the southwestern United States. This reveals large-scale seasonal patterns of the crust contracting and extending in-phase with the Earth's surface going down and up, respectively, particularly in northern California which experiences a large excess of water and snow in late winter. The seasonal variations in horizontal deformation there correspond to variations in the number of mainshocks, with more earthquakes occurring when the crust is under extension. In southern California, we see no correlation with the number of mainshocks. In both regions, seasonal deformation correlates with the proportion of large earthquakes and shows an anticorrelation with the aftershock production. So even though seasonal deformation may not directly trigger earthquakes, if an earthquake happens during the right season, it seems to be able to grow a little larger, releasing a little more stress than it otherwise would and reducing the need for (more) aftershocks.